At present, the predominant trend in aerosol engineering is the development of automatic analyzers of dustiness of gaseous media. Therewith, stringent requirements are imposed on the metrological characteristics of analyzers, particularly, on the particle counting error. This requirement is met by introducing a known quantity of particles into an analyzer, thereby enabling the intrinsic particle counting error of the latter to be determined.
Known in the art is a device for dispensing metered amounts of small-size particles (cf. U.S. Pat. No. 3,926,344; Cl. G01f 11/28), comprising vertical ducts with ports and valves for letting out metered amounts of particles, and vertically arranged measuring tubes having their bottom ends inserted into the vertical ducts and intended for particles to flow therethrough. The measuring tubes determine the measuring volume of particles fo various sizes. The valves arranged above the measuring tubes provide for selective feed of small-size particles into the measuring tubes. The movement of particles in the device and at its outlet is by gravity.
However, in this prior art device, no provision is made for preliminary (prior to letting out) measurement of the number of discrete particles, whereas, in the case where the device is used for calibrating digital aerosol particle counters, this number should be known with a high degree of accuracy.
In addition, the presence of a valve in the outlet duct of the prior art device increases the turbulence of the moving particles, hence, the probability of the same particles being counted several times.
Also known is a device for introducing particles into an analyzer of dustiness of a gaseous medium (cf. USSR Inventor's Certificate No. 321,730; Cl. G01n 15/00), comprising a cell having a vertical feed duct made in its bottom and communicating with the analyzer, and a transparent cover with a microscope positioned thereabove. The device also comprises a luminous flux discriminator with a light source.
However, in this device, one observes in the transparent cell not the particles themselves, but flashes of the light reflected therefrom, which renders particle counting difficult. Moreover, in the prior art device, a plurality of flashes may be seen in the microscope's field of vision, which lowers the accuracy of counting particles.
In addition, both prior art devices lack a transporting medium, which results in an ineffective egress of small particles from the measuring chamber and the outlet duct, which particles are highly adhesive, i.e. the ability of the device to have its measuring chamber and the outlet duct purged from the particles present therein during each operating cycle is poor. Since, in a certain particle size range this factor is of paramount importance, e.g. 25 to 50 microns and smaller, the adhesiveness of particles increases to such an extent that the effective detachment thereof from the surface of the measuring chamber is not only impossible by gravity, but the reguired velocity of the transporting medium (air) is so high that the source of pure air becomes prohibitively cumbersome.
Another disadvantage of the above devices resides in that non-focused outflow of particles from the outlet duct at a low velocity makes the paths of their movement closer to random ones. This introduces an additional error in calibrating analyzers because again the probability of the same particles being counted by the analyzer's sensor several times is high.